[acl2/acl2] af47c9: Disable xref recording in SBCL.

[MattKaufmann]

Branch: refs/heads/master
Home: https://github.com/acl2/acl2
Commit: af47c9e42b336fb0ef3cf6eb158344e40aa1df22
https://github.com/acl2/acl2/commit/af47c9e42b336fb0ef3cf6eb158344e40aa1df22
Author: Matt Kaufmann <kauf...@cs.utexas.edu>
Date: 2026-01-09 (Fri, 09 Jan 2026)

Changed paths:
M acl2-init.lisp

Log Message:
-----------
Disable xref recording in SBCL.

This didn't noticeably affect "make regression-everything" time (using -j 20 on Linux):

;;; Previous (Jan. 7):
97642.058u 6791.411s 1:52:23.62 1548.6% 0+0k 4056816+11973888io 30pf+0w

;;; New:
97399.966u 6992.549s 1:51:44.33 1557.0% 0+0k 3353848+12250952io 31pf+0w

I added the following comment in acl2-init.lisp to explain the change,
which was contributed by Jim White (thanks!).

; The following was contributed by Jim White, who used Copilot on 1/8/2026 to
; suggest the following addition to prevent raw Lisp errors when certifying the
; following with SBCL 2.9.11 on ARM64 (Ubuntu 24.04.3 LTS running in Docker
; container): demos/congruent-stobjs-book, demos/brr-test-book,
; demos/refinement-failure-test-book, demos/brr-free-variables-book, and
; workshops/2023/kaufmann-moore/brr-book. If SBCL bug #2137765 is fixed
; (https://bugs.launchpad.net/sbcl/+bug/2137765), then we might remove this
; change sometime later.



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[MattKaufmann]

Branch: refs/heads/testing